1. Field of the Invention
The present invention relates to a suspension arm having an arm body of an open cross sectional configuration which is open in one direction.
2. Description of the Related Art
As shown in FIG. 14, in an L- or A-shaped suspension arm 100 which forms part of a suspension, the outer end thereof is supported to a wheel carrier side via a ball joint 102, and the corner and inner end thereof are supported to a vehicle body side via bushings 104 and 106, respectively, at the forward position and the backward position of the vehicle. An input load applied through the center (point D) of the ball joint 102 is borne at the center (point A) of the bushing 104 at the forward position of the vehicle as well as at the center (point B) of the bushing 106 at the backward position of the vehicle. Accordingly, the load acting plane P, in which an input load applied to the arm 100 acts, is defined by a plane including points D, A, and B.
The arm 100 is formed by welding together two press-worked plates as shown in FIGS. 8 and 9 or by pressing one plate such that it has an open cross sectional configuration which is open toward the bottom. The arm 100 is formed such that the load acting plane P is interposed between the upper and lower faces of the arm 100.
As shown in FIG. 11, in a front suspension, a steering gear 118 is disposed above and in the vicinity of the arm 100 serving as a lower arm of the front suspension. In order to obtain a sufficiently large engine room space and to make the front suspension compact, it is preferable that the steering gear 118 be disposed as close as possible to the arm 100 (that dimension L between the bottom face of the arm 100 and the steering gear 118 of FIG. 12 be reduced), thereby lowering the position of the steering gear 118. However, as shown in FIG. 8, it is necessary to maintain a predetermined clearance between a tie rod 120 and the arm 100, thereby limiting the closeness between the arm 100 and the steering gear 118. As mentioned above, the load acting plane P is interposed between the top and bottom faces of the arm 100. Therefore, the height of the arm 100 is relatively large, thus restricting the low-level disposition of the steering gear 118. This restricts the desired increase in engine room space and the desired attainment of a compact suspension.
The required clearance between the arm 100 and the steering gear 118 restricts the level of the top face of the arm 100. Also, since a required clearance must be maintained between the steering gear 118 and parts disposed within the engine room, the degree of freedom for the disposition of the top face of the arm 100 is relatively small.
Further, since the arm 100 must be located a predetermined height above the road surface, the position of the bottom face thereof is also restricted. Accordingly, the degree of freedom for the disposition of the bottom face of the arm 100 is also small.
In order to dispose the steering gear 118 as close as possible to the arm 100, the cross section of the arm 100 may be made flat, as shown in FIG. 9. However, so long as the top face of the arm 100 is located above the load acting plane P, the steering gear 118 cannot be disposed at a sufficiently low level.
In order to dispose the steering gear 118 at a sufficiently low level, the top face of the arm 100 may be brought under the load acting plane P. However, in this case, as shown in FIG. 13, the centroid G of cross section of the arm 100 and the shear center S of the arm 100 are located under the load acting plane P. This causes an increase in the offset OS from the load acting plane P of the centroid G and the shear center S as compared with the case where the load acting plane P is interposed between the top and bottom faces of the arm 100, resulting in an increased bending moment caused by an input load applied to the arm 100. As a result, the arm 100 tends to be easily twisted. This causes a strength disadvantage in the arm 100 and reduces the torsional rigidity of the arm 100, which is required for stable travel of a vehicle. To cope with this problem, the cross sectional area of the arm 100 must be increased through, for example, increasing the thickness of the arm 100.
A suspension arm having an open cross section as shown in FIG. 10 is disclosed in Japanese Utility Model Application Laid-Open (JP-U) No. 1-93108. The suspension arm of FIG. 10 has a simple and lightweight structure, but is still poor in efficiency of space utilization, because the load acting plane P is interposed between the upper face and lower end thereof.
In the arm 100 having an open cross sectional configuration, the shear center S can be positioned closer to the closed end thereof (the upper face thereof) as compared with an arm 100 having a closed cross sectional configuration.